1. Field of the Invention
This invention relates to position detection apparatus and methods, and more particularly, to non-contact position detecting apparatus and methods. The related position detection apparatus and methods measure the time response of a transient voltage waveform in an electromagnetic probe (i.e., a probe generating an electromagnetic field) placed in the proximity of an electrically conductive element.
2. Description of the Related Art
The related art discloses several position detection apparatus and methods which use the response characteristics of a transient voltage induced in an electromagnetic probe to measure the position of a conductive element.
U.S. Pat. No. 4,560,929 (Melnyk) discloses an apparatus and method employing damped oscillations to sense the position of an electrically conductive printing band. A low frequency oscillator is used to generate, at periodic intervals, an excitation pulse which induces high frequency oscillations in the tuned coil assembly. The number of oscillatory cycles resulting from a single excitation pulse is a function of the system damping, the system damping being determined by the eddy current losses associated with the metallic printing band moving through the magnetic field of the sensing element. Therefore, the number of oscillatory cycles is representative of the printing band displacement or position. This method of eddy current sensing has the primary disadvantage in that the resolution is limited to a complete oscillation cycle. In order to achieve a high resolution response, it is necessary to set the threshold voltage used for oscillation detection very low, a condition which results in an unacceptably low signal to noise ratio for many applications. A second disadvantage which limits the applicability of this technique, particularly in vibration measurement, is that the response time of the system is slow because a large number of cycles must be processed to derive the positional information.
U.S. Pat. No. 4,843,259 (Weisshaupt) discloses a position detection apparatus and method which evaluates the damped oscillatory response of an inductive probe to a pulse function input by rectifying the signal and evaluating the envelope of the response. This method of proximity detection has the primary disadvantage in that the envelope of the response is subject to errors associated with component parameter variations. The component parameter variations result from changes in ambient temperature typical of analog signal processing circuitry. Other disadvantages of this technique include a slow response time and the relative complexity of the analog to digital processing circuitry used to demodulate the envelope.
U.S. Pat. No. 3,735,229 (Brockmuller) discloses an oscillating servo motor control circuit which periodically energizes a resonant RLC (resistive, inductive, capacitive) tank circuit to measure the degree of damping. The degree of damping is determined by the proximity of an eddy current body. The damped oscillatory response waveform of the resonant tank circuit is rectified, integrated, and compared with a threshold voltage to generate a signal which is used for corrective positioning of the eddy current body. By deriving the positional information from the integral of the tank circuit response, the accuracy of the system has the typical limitations inherent to analog signal processing over varying ambient temperatures. This technique has the additional limitation of an excessively slow response time.